/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package io.netty.util.internal;

import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.lang.Thread.UncaughtExceptionHandler;
import java.security.SecureRandom;
import java.util.Random;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * A random number generator isolated to the current thread.  Like the global {@link
 * java.util.Random} generator used by the {@link java.lang.Math} class, a {@code ThreadLocalRandom}
 * is initialized with an internally generated seed that may not otherwise be modified. When
 * applicable, use of {@code ThreadLocalRandom} rather than shared {@code Random} objects in
 * concurrent programs will typically encounter much less overhead and contention.  Use of {@code
 * ThreadLocalRandom} is particularly appropriate when multiple tasks (for example, each a {@link
 * io.netty.util.internal.chmv8.ForkJoinTask}) use random numbers in parallel in thread pools.
 *
 * <p>Usages of this class should typically be of the form:
 * {@code ThreadLocalRandom.current().nextX(...)} (where {@code X} is {@code Int}, {@code Long},
 * etc). When all usages are of this form, it is never possible to accidently share a {@code
 * ThreadLocalRandom} across multiple threads.
 *
 * <p>This class also provides additional commonly used bounded random
 * generation methods.
 *
 * //since 1.7 //author Doug Lea
 */
@SuppressWarnings("all")
public final class ThreadLocalRandom extends Random {

  private static final InternalLogger logger = InternalLoggerFactory
      .getInstance(ThreadLocalRandom.class);

  private static final AtomicLong seedUniquifier = new AtomicLong();

  private static volatile long initialSeedUniquifier;

  private static final Thread seedGeneratorThread;
  private static final BlockingQueue<Long> seedQueue;
  private static final long seedGeneratorStartTime;
  private static volatile long seedGeneratorEndTime;

  static {
    initialSeedUniquifier = SystemPropertyUtil.getLong("io.netty.initialSeedUniquifier", 0);
    if (initialSeedUniquifier == 0) {
      boolean secureRandom = SystemPropertyUtil.getBoolean("java.util.secureRandomSeed", false);
      if (secureRandom) {
        seedQueue = new LinkedBlockingQueue<Long>();
        seedGeneratorStartTime = System.nanoTime();

        // Try to generate a real random number from /dev/random.
        // Get from a different thread to avoid blocking indefinitely on a machine without much entropy.
        seedGeneratorThread = new Thread("initialSeedUniquifierGenerator") {
          @Override
          public void run() {
            final SecureRandom random = new SecureRandom(); // Get the real random seed from /dev/random
            final byte[] seed = random.generateSeed(8);
            seedGeneratorEndTime = System.nanoTime();
            long s = ((long) seed[0] & 0xff) << 56 |
                ((long) seed[1] & 0xff) << 48 |
                ((long) seed[2] & 0xff) << 40 |
                ((long) seed[3] & 0xff) << 32 |
                ((long) seed[4] & 0xff) << 24 |
                ((long) seed[5] & 0xff) << 16 |
                ((long) seed[6] & 0xff) << 8 |
                (long) seed[7] & 0xff;
            seedQueue.add(s);
          }
        };
        seedGeneratorThread.setDaemon(true);
        seedGeneratorThread.setUncaughtExceptionHandler(new UncaughtExceptionHandler() {
          @Override
          public void uncaughtException(Thread t, Throwable e) {
            logger.debug("An exception has been raised by {}", t.getName(), e);
          }
        });
        seedGeneratorThread.start();
      } else {
        initialSeedUniquifier = mix64(System.currentTimeMillis()) ^ mix64(System.nanoTime());
        seedGeneratorThread = null;
        seedQueue = null;
        seedGeneratorStartTime = 0L;
      }
    } else {
      seedGeneratorThread = null;
      seedQueue = null;
      seedGeneratorStartTime = 0L;
    }
  }

  public static void setInitialSeedUniquifier(long initialSeedUniquifier) {
    ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier;
  }

  public static long getInitialSeedUniquifier() {
    // Use the value set via the setter.
    long initialSeedUniquifier = ThreadLocalRandom.initialSeedUniquifier;
    if (initialSeedUniquifier != 0) {
      return initialSeedUniquifier;
    }

    synchronized (ThreadLocalRandom.class) {
      initialSeedUniquifier = ThreadLocalRandom.initialSeedUniquifier;
      if (initialSeedUniquifier != 0) {
        return initialSeedUniquifier;
      }

      // Get the random seed from the generator thread with timeout.
      final long timeoutSeconds = 3;
      final long deadLine = seedGeneratorStartTime + TimeUnit.SECONDS.toNanos(timeoutSeconds);
      boolean interrupted = false;
      for (; ; ) {
        final long waitTime = deadLine - System.nanoTime();
        try {
          final Long seed;
          if (waitTime <= 0) {
            seed = seedQueue.poll();
          } else {
            seed = seedQueue.poll(waitTime, TimeUnit.NANOSECONDS);
          }

          if (seed != null) {
            initialSeedUniquifier = seed;
            break;
          }
        } catch (InterruptedException e) {
          interrupted = true;
          logger
              .warn("Failed to generate a seed from SecureRandom due to an InterruptedException.");
          break;
        }

        if (waitTime <= 0) {
          seedGeneratorThread.interrupt();
          logger.warn(
              "Failed to generate a seed from SecureRandom within {} seconds. " +
                  "Not enough entropy?", timeoutSeconds
          );
          break;
        }
      }

      // Just in case the initialSeedUniquifier is zero or some other constant
      initialSeedUniquifier ^= 0x3255ecdc33bae119L; // just a meaningless random number
      initialSeedUniquifier ^= Long.reverse(System.nanoTime());

      ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier;

      if (interrupted) {
        // Restore the interrupt status because we don't know how to/don't need to handle it here.
        Thread.currentThread().interrupt();

        // Interrupt the generator thread if it's still running,
        // in the hope that the SecureRandom provider raises an exception on interruption.
        seedGeneratorThread.interrupt();
      }

      if (seedGeneratorEndTime == 0) {
        seedGeneratorEndTime = System.nanoTime();
      }

      return initialSeedUniquifier;
    }
  }

  private static long newSeed() {
    for (; ; ) {
      final long current = seedUniquifier.get();
      final long actualCurrent = current != 0 ? current : getInitialSeedUniquifier();

      // L'Ecuyer, "Tables of Linear Congruential Generators of Different Sizes and Good Lattice Structure", 1999
      final long next = actualCurrent * 181783497276652981L;

      if (seedUniquifier.compareAndSet(current, next)) {
        if (current == 0 && logger.isDebugEnabled()) {
          if (seedGeneratorEndTime != 0) {
            logger.debug(String.format(
                "-Dio.netty.initialSeedUniquifier: 0x%016x (took %d ms)",
                actualCurrent,
                TimeUnit.NANOSECONDS.toMillis(seedGeneratorEndTime - seedGeneratorStartTime)));
          } else {
            logger.debug(String.format("-Dio.netty.initialSeedUniquifier: 0x%016x", actualCurrent));
          }
        }
        return next ^ System.nanoTime();
      }
    }
  }

  // Borrowed from
  // http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/ThreadLocalRandom.java
  private static long mix64(long z) {
    z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;
    z = (z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L;
    return z ^ (z >>> 33);
  }

  // same constants as Random, but must be redeclared because private
  private static final long multiplier = 0x5DEECE66DL;
  private static final long addend = 0xBL;
  private static final long mask = (1L << 48) - 1;

  /**
   * The random seed. We can't use super.seed.
   */
  private long rnd;

  /**
   * Initialization flag to permit calls to setSeed to succeed only while executing the Random
   * constructor.  We can't allow others since it would cause setting seed in one part of a program
   * to unintentionally impact other usages by the thread.
   */
  boolean initialized;

  // Padding to help avoid memory contention among seed updates in
  // different TLRs in the common case that they are located near
  // each other.
  private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;

  /**
   * Constructor called only by localRandom.initialValue.
   */
  ThreadLocalRandom() {
    super(newSeed());
    initialized = true;
  }

  /**
   * Returns the current thread's {@code ThreadLocalRandom}.
   *
   * @return the current thread's {@code ThreadLocalRandom}
   */
  public static ThreadLocalRandom current() {
    return InternalThreadLocalMap.get().random();
  }

  /**
   * Throws {@code UnsupportedOperationException}.  Setting seeds in this generator is not
   * supported.
   *
   * @throws UnsupportedOperationException always
   */
  public void setSeed(long seed) {
    if (initialized) {
      throw new UnsupportedOperationException();
    }
    rnd = (seed ^ multiplier) & mask;
  }

  protected int next(int bits) {
    rnd = (rnd * multiplier + addend) & mask;
    return (int) (rnd >>> (48 - bits));
  }

  /**
   * Returns a pseudorandom, uniformly distributed value between the given least value (inclusive)
   * and bound (exclusive).
   *
   * @param least the least value returned
   * @param bound the upper bound (exclusive)
   * @return the next value
   * @throws IllegalArgumentException if least greater than or equal to bound
   */
  public int nextInt(int least, int bound) {
    if (least >= bound) {
      throw new IllegalArgumentException();
    }
    return nextInt(bound - least) + least;
  }

  /**
   * Returns a pseudorandom, uniformly distributed value between 0 (inclusive) and the specified
   * value (exclusive).
   *
   * @param n the bound on the random number to be returned.  Must be positive.
   * @return the next value
   * @throws IllegalArgumentException if n is not positive
   */
  public long nextLong(long n) {
    if (n <= 0) {
      throw new IllegalArgumentException("n must be positive");
    }

    // Divide n by two until small enough for nextInt. On each
    // iteration (at most 31 of them but usually much less),
    // randomly choose both whether to include high bit in result
    // (offset) and whether to continue with the lower vs upper
    // half (which makes a difference only if odd).
    long offset = 0;
    while (n >= Integer.MAX_VALUE) {
      int bits = next(2);
      long half = n >>> 1;
      long nextn = ((bits & 2) == 0) ? half : n - half;
      if ((bits & 1) == 0) {
        offset += n - nextn;
      }
      n = nextn;
    }
    return offset + nextInt((int) n);
  }

  /**
   * Returns a pseudorandom, uniformly distributed value between the given least value (inclusive)
   * and bound (exclusive).
   *
   * @param least the least value returned
   * @param bound the upper bound (exclusive)
   * @return the next value
   * @throws IllegalArgumentException if least greater than or equal to bound
   */
  public long nextLong(long least, long bound) {
    if (least >= bound) {
      throw new IllegalArgumentException();
    }
    return nextLong(bound - least) + least;
  }

  /**
   * Returns a pseudorandom, uniformly distributed {@code double} value between 0 (inclusive) and
   * the specified value (exclusive).
   *
   * @param n the bound on the random number to be returned.  Must be positive.
   * @return the next value
   * @throws IllegalArgumentException if n is not positive
   */
  public double nextDouble(double n) {
    if (n <= 0) {
      throw new IllegalArgumentException("n must be positive");
    }
    return nextDouble() * n;
  }

  /**
   * Returns a pseudorandom, uniformly distributed value between the given least value (inclusive)
   * and bound (exclusive).
   *
   * @param least the least value returned
   * @param bound the upper bound (exclusive)
   * @return the next value
   * @throws IllegalArgumentException if least greater than or equal to bound
   */
  public double nextDouble(double least, double bound) {
    if (least >= bound) {
      throw new IllegalArgumentException();
    }
    return nextDouble() * (bound - least) + least;
  }

  private static final long serialVersionUID = -5851777807851030925L;
}
